Power MOSFETs are a type of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) that is designed to handle significant power levels. Some of these devices are designed to switch high currents and to have low on resistance. Some of these devices are designed to tolerate high voltages across the device's terminals. The voltage tolerance and current requirements have resulted in device configurations different from tradition MOSFET designs. One such device configuration involves trenches, which have been used to provide vertical channel conduction for such power MOSFETS. Orienting these MOSFETS vertically has improved the layout efficiency of such devices.
These high-voltage trench MOSFETS can have a backside or substrate terminal connection (e.g., drain connection). When providing electrical connection to the drain via the substrate, much of the semiconductor die will be biased in response to the voltage applied to the substrate. This bias voltage will be electrically communicated from the backside contact surface up throughout much of the die. The bias voltage will be electrically transmitted where the conductivity is high and where no semiconductor junctions are encountered. These high voltages should be controlled so that undesirable breakdowns do not occur.